1. Field of the Invention
This invention relates to a coating for use on mirror backs, and, in particular, to a lead-free organic coating containing a di- or tri- nitrile containing compound, preferably iminodiacetonitrile or nitrilotriacetonitrile, which coating is applied to the metallic film layer on the back of a mirror to protect the metallic layer and prevent corrosion thereof.
2. Description of Related Art
Typical mirrors are made of a glass sheet and a thin layer of a metallic film applied to the back of the sheet. The metallic film layer adhered directly to the glass is usually a film of silver, although other metallic films may also be used, such as copper. When silver is used as the primary reflective layer, it is commonly protected by a second metallic film layer of copper or some other metal.
It has long been known to employ various paints and other film-forming organic resins as a further protective layer over a metallic film layer to protect the layer from corrosion and physical damage. Traditionally, these paints have included lead-based corrosion inhibitors, such as lead salts. However, in recent times, both users and producers of such paint products have sought to eliminate the use of lead and lead compounds for health and environmental reasons.
Numerous efforts have been made to eliminate lead from paints and/or otherwise increase the effectiveness and usefulness of protective coatings for mirror back metallic films. A recent effort in this direction is reflected in U.S. Pat. No. 4,707,405 to Evans et al. directed to use of cyanamide salts of non-lead metals as corrosion inhibitive pigments in mirror back coatings. This patent discloses the use of such non-lead cyanamide salts as calcium cyanamide and zinc cyanamide in various types of film-forming thermosetting or thermoplastic resins which are applied over the silver and copper layers on the backs of mirrors. U.S. Pat. Nos. 5,248,331 and 5,252,402 to Sanford et al. disclose a lead-free paint or resin composition for use as a mirror back coating containing as a corrosion inhibitor dicyandiamide, metal or acid salts thereof, hydrogen cyanamide and 2-cyanoacetamide. U.S. Pat. No. 5,314,532 to Hughes et al. discloses the use of soluble salt-free contaminant-free antioxidant pigments which are cyanamide derivatives of metals and which are used in combination with organic resin polymers as protective mirror backing coatings. The patent discusses the prior art U.S. Pat. No. 4,707,405 to Evans et al., supra, which uses the same cyanamide salts of non-lead metals as a corrosion inhibitor in organic film forming resins for mirror back coatings with the improvement being the use of "pure" salts. Accordingly, the pure salts contain less than about 0.5 percent by weight of soluble salts. U.S. Pat. No. 4,255,214 to Workens discloses a method for protecting silver and copper coated mirrors against oxidation and corrosion by passivating the metallic coating prior to applying a protective organic coating. The silver and copper coating is passivated by applying an effective amount of tolyltriazole to the metal surfaces before applying the protective organic coating. U.S. Pat. No. 5,389,301 to Fenzi discloses a lead-free anticorrosive resin formulation containing cyanoacetylurea for protecting mirror backs. Japanese patents 5311485, 5311492 and 5311495 show processes for treating a noble metal, including silver, by electrolyzing the metal in an aqueous solution containing an organic compound such as an imino compound or other nitrogen containing compounds such as EDTA to increase the corrosion resistance of the noble metal.
U.S. Pat. No. 4,543,215 to Brunnmueller et al. discloses novel iminodiacetonitriles and their preparation. It is disclosed that the nitrites are useful as starting materials for the preparation of dyes, fungicides and other materials including anticorrosion agents.
The disclosures of the above patents are hereby incorporated by reference.
Bearing in mind the problems and deficiencies of the prior art, and the long felt need of industry, it is therefore an object of the present invention to provide an effective corrosion inhibiting, lead-free coating for covering the metallic film layers on the back of a mirror and other articles of manufacture.
It is another object of the present invention to provide an organic film-forming resin which incorporates a non-lead corrosion inhibitor which can be easily applied by existing techniques to mirror backs.
It is a further object of the present invention to provide a lead-free paint for protecting thin silver and/or copper or other metallic film layers against corrosion.
It is yet another object of the present invention to provide an effective process for inhibiting corrosion of metallic film layers on mirrors and other articles of manufacture.
It is a further object of the present invention to provide mirror and other articles having effective protection of their metallic film layers against salt spray and other corrosion-causing compounds.
Other objects and advantages of the present invention will be readily apparent from the following description.